Casting molten metal



T. J. BIRCHILL ET L 3,494,410

Feb. 10, 1970 CAST ING MOLTEN METAL 2 Sheets-Sheet 1 Filed Dec. 12, 1966 m ON INVENTOR. THEODORE J. BIRCHILL THOMAS E. GROCE g. ATTORNEY 1. J. BIRCHILL ET A 94,

Feb. 10, 1970 CASTING MOLTEN METAL 2 Sheets-Sheet 2 Filed Dec. 12, 1966 lN V ENTOR. THEODORE J. BIRCHILL BYTHOMAS EGROCE ATTORNE United States Patent 3,494,410 CASTING MOLTEN METAL Theodore J. Birchill and Thomas E. Groce, Spokane,

Wash., assiguors to Kaiser Aluminum & Chemical Corporation, Oakland, Calif., a corporation of Delaware Filed Dec. 12, 1966, Ser. No. 600,929 Int. Cl. B22d 11/00 US. Cl. 164-82 3 Claims ABSTRACT OF THE DISCLOSURE A continuous casting process wherein molten aluminum is transferred under flow conditions such that the protective layer of dross is maintained in a static condition thereby substantially eliminating the inclusion of dross into the molten aluminum.

This invention relates to continuous casting of metal, and particularly to a method for casting metal that results in clean and inclusion-free ingots.

Although the method of this invention may be used to cast any molten metal, it is particularly useful for casting aluminum, and although not limited to use in casting aluminum it will be dscribed with reference to that metal. Molten aluminum quickly oxidizes and a floating oxide layer called dross forms on the molten metal. The dross layer protects the molten metal beneath from further oxidation, but individual particles of aluminum oxide in the dross layer are very small and a quiescent time period is required for the dross particles and the aluminum to separate gravimetrically. Particularly under the turbulent conditions of pouing molten aluminum into molds, oxide particles are formed and entrained in the molten metal and trapped in it as it solidifies thereby forming inclusions in resultant ingots. These inclusions interefere with the soundness of ingots and cause imperfections in wrought products that are subsequently made from them.

In most casting processes molten aluminum is poured into molds and the splashing that results exposes a great deal of surface to oxygen and it creates an extremely turbulent condition that causes oxide particles to become entrained in the molten metal. However, even less violent casting procedures result in oxide inclusions because turbulence in a flowing metal stream causes small oxide particles to be drawn into the flowing stream from the floating dross layer. It is the primary object of this invention to provide a metal casting process that avoids introducing oxide inclusions into ingots.

Other advantages obtained from the process of this invention include that the operation of the molds is very simple. One level control in the trough feeding the molds is all that is required and as such no level control equipment need be maintained in the mold. This not only represents a reduced amount of total equipment, but it takes equipment out of the mold which is a crowded area, and it prevents interference with the solidification pattern in the mold, which not only promotes soundness of the resultant ingot, but also prevents segregation of alloying elements by fractional crystallization around the cooler control devices. The invention also eliminates spouts, distribution pans, and other devices from the mold.

The process of the present invention includes a large supply or capacity of molten aluminum such as a melting or holding furnace that provides molten aluminum to a supply trough that feeds molten aluminum to at least one continuous casting mold, and preferably to a plurality of continuous casting molds. The metal flows from the furnace through the supply trough and into the continuous casting mold from which a solid ingot is withdrawn. The primary criterion of the process is that the trough and all ice of the casting stations are at substantially the same molten metal level. Operating in accordance with this criterion results in a process, as will be described more fully hereinafter, that substantially eliminates introduction of inclusions into ingots.

The operation of the system requires attention to two separate flow rates. The first of these is the sum of the flow rates of metal into all of the casting molds. The flow rate of metal to any mold is related to the drop rate, or the rate at which solid ingot is withdrawn from the mold. It is apparent that the flow rate of molten metal through the trough serving the various molds is related to the collective or total drop rate or the sum of all flow to all molds fed from the trough. It is essential in this invention that the capacity of the trough as compared with the collective drop rate be such that the level of molten metal in all portions of the trough is the same. It is, of course, realized that in order for liquid to flow by gravity it is essential that a difference in head be maintained between its starting point and its destination, and accordingly some difi'erence in level between the point where the metal enters the trough and the various points where the metal discharges from the trough will exist. However, it is the essence of this invention that the capacity of the trough compared with the flow rate through it be large enough so that the difference in level between various points in the trough is so small as to be substantially imperceptible.

The second flow rate that requires attention is the rate of flow of molten metal from a trough through a passageway into a mold. This flow rate is controlled by the drop rate of the individual mold involved, and it must be slow enough so that the fixed size passageway between the trough and the mold can pass metal into the mold at the same rate that it is being withdrawn as solid ingot without causing a perceptible difference in liquid level between the mold and the trough, and here again the necessity for some difference in level is recognized.

When these criteria are established the flow of liquid metal occurs beneath a static layer of dross on the surface. The flow rate under conditions of constant level at all points in the system is so slow that it is not turbulent enough to draw dross particles down from the floating layer into the flowing stream. The floating layer of dross, in effect, forms the upper surface of a conduit through which the liquid metal flows, the other portions of the conduit being the floor and walls of the trough or the walls of the continuous casting mold.

The molten metal in the mold is withdrawn into the casting zone by withdrawing solid ingot from the bottom of the volume of metal that is in the mold. The molds must employ no spouts or free falls through the atmosphere so that the metal is not contaminated with oxide particles in passing from the liquid state into the solid state. The molds of this invention operate by withdrawing molten metal through the mold from beneath the surface of a liquid metal pool.

The term trough is used in this description to designate a metal transfer device, usually open-topped, to carry molten metal from a holding furnace to a continuous casting mold. Although it is contemplated that the trough will be of elongated configuration, it is within the scope of this invention to use a trough that is square or round in shape. It is also contemplated that the trough can contain devices to perform functions in addition to transferring metal. For example, the metal transfer trough may contain a filter for removing inclusions already entrained in the molten metal stream for example by erosion of refractories, or a stripping gas tap to remove dissolved gas from the molten metal stream.

The process of the present invention can be best described with reference to the accompanying drawings,

FIGURE 1 illustrates an embodiment of the invention in Schematic elevation view in cross section taken along the line 1-1 of FIG. 2, showing a holding furnace, a trough, and a group of casting molds.

FIGURE 2 shows a schematic plan view of the apparatus of FIGURE 1.

The process of this invention, as illustrated in the figures, is begun by introducing aluminum into refractory lined furnace where it is either melted or maintained molten by conventional heating means which are not shown. The body of molten aluminum 11 is protected from the atmosphere by a floating layer of dross indicated as 12. The furnace is preferably maintained closed for a suflicient period of time for substantially all of the dross to separate from the molten metal gravimetrically after which the opening 13 is opened and molten aluminum is permitted to flow into refractory lined trough 15. The furnace may have provisions for stripping gas or filtering, which are not shown. In trough 15 a body of molten aluminum 16 is maintained upon which a layer of dross 17 floats, and the molten aluminum in the trough passes through passageways 20 into molds 21 having cooled means 22 for casting ingots 23.

Within each mold 21 there also is maintained a layer of molten aluminum upon which a protective layer of dross is floating. A valve-like device 18 is also illustrated to show means for opening and closing the port 13 and, if desired to maintain, either manually or automatically, a constant level in trough 15. Although a controlled level in trough 15 is desirable, it is not necessary in the operation of this invention. Minor variations in level can be tolerated as long as the level in trough 15 does not vary from one part of the trough to another.

The volume of trough 15 is designed so that the mass flow rate through the trough 15 can be sufficient to supply all of the molds 21 without a perceptible difference in level between entry port 13 and the opposite end of the trough. Also, ports 20 providing passageways into molds 21 must be large enough so that the molten metal flowing into molds 21 can flow at a sufficient rate to supply metal to the ingots 23 without a perceptible difference in level between the molten metal in molds 21 and the molten metal in trough 15.

Once the trough 15 and the molds 21 are filled, the area of the surface of molten aluminum never changes and, in fact, in operation the trough and mold tops appear as a static system. The only flow is beneath the surface of the dross and hidden from view. As long as the trough 15 has suflicient capacity there is no need for all of molds 21 to be the same size or to produce ingots of the same cross sectional area or shape. It is within the scope of this invention to substitute one mold .21 for another to produce ingots of different shapes and different sizes, and the only limitation on the shape and size of an ingot produced in the process is that the drop rate in the specific mold employed should not be so great that the opening 20 is too small to maintain substantially the same level both in the mold and in the trough, or that the drop rate is not so great that it causes the collective drop rate to produce a perceptible difference in level from the entry side to the exit side of the trough 15. Although the passageways 20 through the side of the trough 15 and into the molds are shown as being beneath the interface between molten aluminum 16 and dross 17 the passageways can include the interface and for that matter intersect the upper edge of trough 15. When the passageways 20 are so arranged, a layer of dross will float within each opening and the flow of aluminum will take place beneath that layer. The passageways 20 may be round, oval, nozzle-like or of any other shape to promote whatever flow pattern is desired in the mold. Passageways 20 may also be through the floor of trough 15 and two or more passageways may be used to supply metal to a mold.

The molds 21 are illustrated quite schematically showing substantially none of the elements normally used for such molds. Casting equipment normally includes a short mold with extremely high cooling rate so that the molten metal solidifies in a short period of time and at a controlled rate as it is drawn through the mold. These molds may have provision for lubricant, cooling Water, cutting of ingots, etc. The ingots shown in the drawings are being cast vertically, but ingots can be cast horizontally or in any other manner within the scope of this invention. The only limitation on the molds of this invention is that the mold feed is withdrawn from a substantially constantvolume pool of metal into which metal is introduced as a non-turbulent stream. The molds employed in this invention may be of the type known as hot top molds in which the upper portion of the mold is insulated to maintain a molten metal reservoir for the ingot to draw from, or other types of mold tops may also be used. The term mold, in this description includes the tops and mold extensions and is not limited to the cooled portion where metal solidifies.

The drawings also illustrate schematically a control system that may be used to maintain the casting process at steady conditions. The drawings show a level sensing device shown as a float in trough 15 which senses the height of the level in the trough and transmits a suitable impulse, such as mechanical, electrical, or pneumatic signals to controller 31. Controller 31, acting responsive to signals representative of the location of the level, operates valve 18 to increase or decrease the flow of aluminum from furnace 10 in order to maintain the liquid level in trough 15 constant. As the liqhid level in trough 15 becomes too high, or higher than a set point level, the valve 18 will close slightly and if the level in trough 15 drops below the set point level, the valve 18 will open slightly.

Also illustrated as one means for controlling flow rate is a differential level sensing device shown as 33, which senses the level in trough 15 through float and the level in mold 21 through float 41. The controller 33 acts responsive to the differential in level to vary the drop rate of ingot 23 by varying the speed of motor 37 which controls the rate at which rollers 36 rotate to Withdraw ingot 23. Conductor 38 is shown as a typical means for transmitting signals between sensing means and controllers, and between controllers and servomechanisms used to effectuate the control. Conductor 38 connects the differential level control 33 with the motor 37 so that the drop rate will vary responsive to the level differential measured at 33. The differential in level between the metal in trough 15 and in mold 21 is maintaind to not exceed a certain set point value, because in this case, the level in mold 21 will always be equal to or lower than the level in trough 15. When the level differential becomes too great, the drop rate will be diminished in order to allow the flow through opening 20 to equal the amount of aluminum being withdrawn from the mold with a lesser difference in head between the trough and the mold. The drop rate in all cases must be sufficient to provide an operable process, and the control of drop rate by differential level sensing is contemplated to be an override to prevent turbulent flow conditions rather than a regulator to establish processing conditions.

Also illustrated as an optional feature of this invention is a filter 45. The filter is placed so that all metal flowing from furnace 10 to molds 21 must pass through it thereby removing entrained solids picked up at any prior point in the process. The filter 45 may also be placed inside of furnace 10 or at any other location in the system. When filter 45 causes a level differential due to pressure drop in flowing through it, the constant level of the trough is from the filter 45 to the molds 21. A level differential across filter 45 is not the type of level difference to be avoided because it causes no turbulent flow and because the level differential across the filter contributes to removal of inclusions rather than introducing them.

As a specific example of the operation of the process of this invention, a body of aluminum metal is maintained in furnace 10 and the temperature is controlled at 1320 F. The molten metal trough 15 may be provided with electric heating elements in the bottom and sides and when the molten metal in furnace 10 is heated to temperature and maintained quiescent for a period of at least one-half hour, the refractory lining of the trough is heated to 1320 F. and valve 18 is opened so that metal may flow through opening 13. Opening 13 is a round opening with a diameter of about three inches and it discharges metal into a trough having a cross section in the form of an open-topped rectangle with a bottom eight inches in length and side walls extending upwardly ten inches from the bottom. As stated heretofore, the trough is refractory lined. The openings in the side wall of the trough are squares 1.75 inches on a side and they discharge into an insulated mold top that feeds a mold capable of casting an ingot having a square cross section with sides ten inches long. The openings 20 into the mold are two inches above the floor of the trough as is the opening 13 from the furnace 10.

Drop rates between two and three inches per minute may be maintained easily without creating a level differential, either from the inlet to the outlet of the trough, or between the metal surface in the mold and the metal surface in the trough. Drop rates between two and three inches per minute are established by considerations of metallury rather than to maintain levels.

The ingots are cast by withdrawing molten metal from the bottom of the molds and the ingots thus cast are exceptionally sound and substantially free of oxide incluslons.

Of course, the initial portions of the ingot, made from metal passing into the mold before steady state conditions are attained, may contain aluminum oxide inclusions and that metal must be discarded as scrap, but once steady state conditions of casting are obtained a continuous desirable product having the sought absence of inclusions is obtained as long as a molten metal supply is continued.

What is claimed is:

1. In a process for the continuous casting of molten aluminum wherein a supply of molten metal of relatively large volume is maintained, molten aluminum is introduced from beneath the surface of molten aluminum in a trough, and at least one mold is supplied with molten aluminum from said trough through a passageway between the trough and mold, the improvement comprising transferring the molten aluminum through the trough and mold under flow conditions such that a protective layer of dross is maintained in a static condition, substantially undisturbed by the transferring molten aluminum, thereby substantially eliminating the inclusion of dross into the transferring molten aluminum.

2. The process of claim 1 wherein the molten metal is filtered between the supply and the first mold fed from the trough.

3. The process of claim 1 wherein the molten metal is transferred from the trough to a mold through a passageway in a sidewall of said trough.

References Cited UNITED STATES PATENTS 2,195,809 4/1940 Betterton at al. 164--82 2,397,789 4/1946 Hopkins 164133 2,840,871 7/1958 Gaffney 164134 FOREIGN PATENTS 1,230,089 3/1960 France.

891,444 9/1953 Germany.

r J. SPENCER OVERHOLSER, Primary Examiner 0 R. S. ANNEAR, Assistant Examiner US. Cl. X.R. 

